Background of div is not going with content

[cowgirl]

I am new to html and css. I coded a html page with css but confused.I used this css code\[code\]#container {background: #000000;width: 500px;margin: auto;}#left {background: #FF0000;width: 200px;float: left;}#right {background: #0000FF;width: 200px;float: right;}\[/code\]and this html code\[code\]<div id="container"><div id="left">This is left</div><div id="right">This is right</div></div>\[/code\]But I didn't got black background that I specified in #container.Can you help me with that. I want background to move automatically as i write content. in between divs having id container.